1. Field of the Invention
The present invention concerns an antenna arrangement for a magnetic resonance apparatus of the type having a first antenna group, which comprises at least one antenna element, and a second antenna group, separate from the first antenna group, which likewise comprises at least one antenna element. The invention also concerns a method to couple two antenna groups that are separate from one another to acquire magnetic resonance signals, wherein the antenna groups each include at least one antenna element.
2. Description of the Prior Art
In the magnetic resonance examination of specific organs or body parts of a patient, surface antennas are increasingly used to receive the nuclear magnetic resonance signals (magnetic resonance signals). These surface antennas are arranged in the examination directly at the organ or body part of the patient to be examined, relatively close to the surface of the body. In contrast to larger antennas arranged farther from the patient that are normally used to generate an overall cross-section through a patient, these surface antennas have the consequent advantage of being able to be located closer to the region of interest. The noise component caused by the electrical losses within the body of the patient is thereby reduced, which causes the signal-to-noise ratio (SNR) of a surface antenna, in principle, to be better than that of a farther removed antenna. A disadvantage is that an individual surface antenna is only able to generate an effective image within a predetermined spatial dimension, which lies in the order of magnitude of the diameter of the conductor loop of the surface antenna. The possibilities for use of such individual surface antennas are therefore very limited, due to the restricted region of observation. The region of observation can be expanded, by enlarging the diameter of the conductor loop of the surface antenna, but the enlargement of the conductor loop causes the same with an increase in the aforementioned electrical losses in the body of the patient and, as a consequence the signals are received with an increased noise component. Given use of an individual surface antenna, a compromise path must always be made between the best possible resolution on and the largest possible region of observation.
It is possible to enlarge the region of observation without reducing the resolution to the same degree by using a number of individual surface antennas arranged adjacent to one another, i.e. an entire field of antenna elements, which together form a large surface antenna. A problem of such antenna arrangements with a number of adjacent antenna elements is that a high-frequency current in an antenna element can induce a voltage in an adjacent antenna element, due to inductive coupling. This means that a signal generated in one of the adjacent antennas automatically also causes a signal component in an adjacent antenna element. The inductive coupling consequently degrades the signal-to-noise ratio. In addition, the complexity in the evaluation of the signals from coupled antenna elements is greater than in non-coupled antenna elements. An inductive coupling of the antenna elements therefore should be avoided if possible.
A method to decouple adjacent antennas is disclosed in U.S. Pat. No. 4,825,162, for example. The decoupling is achieved by overlapping the conductor loops of adjacent antennas to a certain degree, such that overall the inductive coupling between the affected antennas is minimal. Due to the mandatory overlap between the involved antenna elements, this decoupling method is not suitable in all cases to decouple adjacent coils.
For example, one such case exists when the adjacent antennas belong to different antenna groups, for example various antenna arrays that are located in separate housings. In a magnetic resonance examination the size of the antenna should be adjusted to the size of the subject to be examined or to the type and the purpose of the examination, a number of different-sized surface antennas have to be available at a magnetic resonance device, with an antenna having the suitable size for a specific examination being selected. In order to limit the number of antenna sizes that must be left at hand, and in order to achieve a greater variability and better adaptation for various uses, antenna arrays are in practice frequently assembled in the form of modules. Each antenna module has a housing with an antenna group, including a number of individual antennas, which form a small antenna array. By joining the antenna modules, antenna arrays of different sizes can be fashioned. A problem is that the antenna elements of the joined antenna groups (that are respectively located at the ends of the antenna groups arranged to one another) are arranged in very close proximity to one another and therefore would inductively couple into one another.
Conventionally, the housings for the antenna groups are fashioned at the edges with fitting pieces, such that both adjacent edge antenna elements overlap, given a coupling of the housings. This is shown in FIG. 1. A first antenna group with a number of individual antenna elements A arranged in a first antenna plane E1 is located in a first housing G1. A second antenna group, which has a number of individual antenna elements A arranged in a second antenna plane E2, is located in a further housing G2. The ends of both housings G1, G2 are fashioned such that, given a coupling of the housings G1, G2 with an accurate fit, the edge antenna elements of both antenna groups inevitably overlap in a suitable manner in order to achieve a decoupling between the adjacent edge antenna elements. As FIG. 1 also shows, the individual antenna elements A of both antenna groups are respectively arranged in different antenna planes E1, E2. This means that one of the antenna groups has a larger separation from the patient than the other antenna group, such that the signal-to-noise ratio of this antenna group is in principle worse. In addition, the housings G1, G2 are each relatively thick, so that a predetermined safety spacing is maintained from the antenna group arranged next to the patient.
A simple placement, one atop the other, of antenna groups that are assembled in simple housings without the fitting piece ends would lead to a widening of the antenna arrangement in the overlap region. Such an arrangement is therefore possible in all cases in examinations in which the antennas are placed upon the patient, but not in arrangements in which the patient is placed upon the antennas.
FIG. 2 shows an alternative that also is presently used in practice to couple two antenna groups. Here the separation ensues between the different housings G1, G2 by means of a common boundary antenna whose conductor paths respectively terminate in a plug contact S at the edges of the housings G1, G2. Given coupling, both halves of the boundary antenna are electrically connected with one another via the plug contact S, such that overall a continuous antenna array is formed. A problem in the use of such plug connections is that the electrical contacts can foul and wear. In particular, it can also become a problem if fluid reaches the plug contact S, which cannot be completely ruled out in the medical field.